Just thought about it while having a whiskey & soda.......
Ben is on track as well......the chambers from the old days were whatever the maker thought worked. There is every possibility that a simply resized and trimmed to length 577 3" would chamber in an original .577-.500 no2 if the chamber is even a tiny bit oversized in the base area.
Like I mentioned; no SAAMI or CIP in the 1800's
- Mike